इन्द्रमुपगतोऽनुजत्वेनेति उपेन्द्रः; यद्वा, उपरि इन्द्रः उपेन्द्रः ।'ममोपरि यथेन्द्रस्त्वं स्थापितो गोभिरीश्वरः ।उपेन्द्र इति कृष्ण त्वां गास्यन्ति भुवि देवताः' इति हरिवंशे ।बलिं वामनरूपेण याचितवानिति वामनः ।संभजनीय इति वा, 'मध्ये वामनमासीनं विश्वे देवा उपासते' इति
In plain words
Upendra: the one who came to Indra's side as his younger brother. Or read the prefix the other way — the Indra above Indra. The Harivaṃśa puts it in Indra's own mouth: "The cows have installed you above me as Indra, Lord; and the gods on earth will sing of you as Upendra, Kṛṣṇa." Vāmana: he took the form of a dwarf and asked Bali. Or Vāmana as the one to be worshipped with everything you have — "all the gods worship the Vāmana seated at the centre."
Close to the Sanskrit
"He who approached Indra by way of being the younger brother" — thus Upendra. Or else: the Indra who is above — Upendra. "Over me you were established as Indra by the cows, O Lord; and as 'Upendra,' O Kṛṣṇa, the deities on earth will sing of you" — thus in the Harivaṃśa. "He who, in the form of a dwarf, made his request of Bali" — thus Vāmana. Or, "he who is to be worshipped wholly": "All the gods worship the Vāmana seated in the middle" — thus.